A material formed from GaN, AlN, or InN, which is a nitride semiconductor, or a mixed crystal thereof or the like has a wide band gap and has been used as a high output electronic device, a short wavelength light-emitting device, or the like. Among them, as for the high output electronic device, a technology related to a field effect transistor (FET), in particular a high electron mobility transistor (HEMT), has been developed. The above-described HEMT by using the nitride semiconductor is used for high-output high-efficiency amplifiers, high-power switching devices, and the like.
HEMT used for such purposes is required to be normally off, have a high dielectric strength, and the like. In particular, normally off is important from the viewpoint of safe operation. Therefore, various methods for ensuring normally off have been studied. As for one of the methods for ensuring normally off, a method in which a gate recess is formed by removing a part of a semiconductor layer immediately below a gate electrode is mentioned. The gate recess structure formed by this method has advantages that, for example, a threshold voltage may be made positive without increasing a resistance component between electrodes. Meanwhile, a semiconductor device of normally off, which is used for electric power applications, is required to have a high drain breakdown voltage and a high gate breakdown voltage. Therefore, regarding horizontal structure FET and HEMT, a metal insulator semiconductor (MIS) structure, in which an insulating film serving as a gate insulating film is formed, has been used. As described above, regarding HEMT by using a GaN based semiconductor material, a semiconductor device suitable for the electric power applications may be produced by employing a structure, in which the gate recess structure and the MIS structure are combined.
Japanese Patent Laid-Open No. 2002-359256 is noted here as related literature.